1. Field of the Invention
The present invention generally relates to a method of manufacturing a non-single-crystal thin film solar cell that is composed mainly of a non-single-crystal film and has a pin or pn junction structure.
2. Background
A thin film solar cell with non-single-crystal thin film, more particularly, a non-single-crystal thin film solar cell, such as a silicon non-crystalline thin film, having a pin junction structure comprised mainly of non-single-crystal thin film such as an amorphous silicon (hereinafter referred to as a-Si), a silicon comprised mainly of microcrystalline phases (hereinafter referred to as xcexcc-Si) (this silicon partially includes a-Si film), and a thin film polycrystalline silicon, can be produced with a larger area at lower temperature and lower cost, as compared to a single-crystal solar cell. This non-single-crystal thin film solar cell is expected to be useful as a large-area thin film solar cell for supplying power.
However, the efficiency of the solar cell comprised of a-Si is lowered, from the beginning, due to the so-called Steabler-Wronski effect, in which the efficiency of the solar cell decreases when exposed to irradiation with light for a long period of time. With respect to this problem, it has recently been reported that it is possible to produce a solar cell with no light-induced degradation by using xcexcc-Si as a p-type semiconductor layer (hereinafter referred to as a p layer) as the doping layer of a pin-type solar cell, an n-type semiconductor layer (hereinafter referred to as an n layer), and a substantially intrinsic i-type high resistivity layer (hereinafter referred to as an i layer). See pp. 3 of J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, A. A. Anna Selvan, N. Pellaton Vaucher Ch. Hof, D. Fischer, H. Keppner, A. Shah, K. D. Ufert, P. Giannoules, J. Koehler, Mat. Res. Soc. Symp. Proc. Vol. 420, 1996.
Unlike the a-Si film, the conductivity of the xcexcc-Si film is never degraded by light irradiation. Thus, it can be considered that the use of the xcexcc-Si for the solar cell inhibits light degradation.
The light absorption coefficient of the xcexcc-Si is small on the short wavelength side and is large on the long wavelength side. The use of the xcexcc-Si for the i layer, which acts as a light absorbing layer, in the pin type solar cell enables the use of long wavelength light and increases the short-circuit current density (hereinafter referred to as Jsc).
On the other hand, the use of xcexcc-Si for the p layer or the n layer on the light incidence side enables an increase in Jsc, due to the reduced light absorption loss on the short wavelength side. Moreover, the increase in diffusion potential improves the open-circuit voltage (hereinafter referred to as Voc). The use of xcexcc-Si for the p layer or the n layer on the side opposite to light incidence increases the Voc, due to an increase in the diffusion potential. The use of xcexcc-Si also increases the fill factor (hereinafter referred to as FF) and Jsc, due to a reduction in contact resistance against the substrate electrode. The formation of a tunnel junction layer by laminating together two or more layers also increases FF and Jsc.
However, in some conditions for producing the xcexcc-Si film, an a-Si film may be formed at an initial stage of the film formation. FIG. 15 is a conceptual drawing of a transmission electron microscope (hereinafter referred to as TEM) photograph showing a section of a thin film solar cell that has been manufactured by laminating an n layer, an i layer and a p layer, in this order, at a substrate temperature of about 250xc2x0 C. The magnification is about two hundred thousand. Although the conditions suitable for forming microcrystals during film formation are selected, an a-Si film is formed at the initial stage of the n layer formation, and an a-Si film is formed in some parts of the i layer.
The efficiency (hereinafter referred to as Eff) of this thin film solar cell was as low as 2.1%. The a-Si film at the initial stage of the film formation covered several 100 nm under some production conditions. It is therefore impossible to form the xcexcc-Si film with the designed thickness.
The formation of even a thin a-Si film at the initial stage of the film formation increases defects and lowers the conductivity. This increases a resistance loss and lowers FF and Jsc. Moreover, the defect density of an interface between the a-Si film and the i layer of the xcexcc-Si film increases to cause further deterioration of cell characteristics. If the layers are formed on a transparent substrate, sequentially from the light incidence side, the Jsc is lowered due to the large absorption coefficient of the initial a-Si film.
In an attempt to inhibit the formation of the non-crystalline film at the initial stage of film formation and to produce the xcexcc-Si film (including microcrystals) on the i layer of a-Si from the beginning, the surface of the i layer is processed by a hydrogen plasma before the xcexcc-Si film is formed. The effect of this method, however, has not yet been proved.
According to a report of Pellaton et. al., it is possible to form an n layer of xcexcc-Si with a coating thickness of not greater than 10 nm for the purpose of forming a tunnel junction layer of a tandem cell by processing the surface of an i layer of a-Si with a carbon dioxide (CO2) plasma. See pp. 651 of N. Pellaton Vaoucher, B. Rech, D. Fischer, S. Dubail, M. Goetz, H. Keppner, C. Beneking, O. Hadjadj, V. Shkllover and A. Shah, Technical Digest of 9th Int. Photovoltaic Science and Engineering Conf., Miyazaki, Nov. 11-15, 1996.
This method, however, lacks controllability and repeatability, since it is difficult to control the composition, coating thickness, etc., of the layers that are formed on the interface by the CO2 plasma. The above publication mentions the use of the a-Si layer as a substrate, but fails to disclose whether it is possible to use glass, a metal electrode and a transparent electrode as the substrate.
The inventor and his co-authors have used a pin-type cell, in which xcexcc-Si was used for the p layer, and a p/i interface layer of an amorphous silicon oxide (a-SiO) was provided at an interface between the p layer and the i layer. According to their report, forming the p layer of xcexcc-Si at a low temperature of about 85xc2x0 C. prevents the formation of an a-Si film at the initial stage of the film formation and improves Voc, as compared with the case where a-SiO is used for the p layer. See T. Sasaki, S. Fujikake, K. Tabuchi, T. Yoshida, T. Hama, H. Sakai and Y Ichikawa, J. Non-Cryst. Solids, to be published; T. Sasaki, S. Fujikake, K. Tabuchi, T. Yoshida, T. Hama, H. Sakai and Y Ichikawa, Tech. Digest of 11th Int. Photovoltaic Science and Engineering Conf., Sapporo. Japan, Sep. 20-24, 1999, to be published. They have also proposed forming the p layer of the xcexcc-Si film with no initial a-Si film and setting the thickness of the a-SiO in the p/i interface layer within an appropriate range to develop a thin film solar cell with a higher efficiency than the case in which a-SiO is used for the p layer. The above structure, however, is only applicable to the case where the a-SiO is formed as an interface layer on the a-Si film. It is not applicable to the case where the substrate is formed of glass, a metal electrode or a transparent electrode at a low temperature.
The inventor has now conducted the same experiment with the condition that the substrate was formed of glass, a metal electrode or a transparent electrode. FIG. 16 is a conceptual drawing of a TEM photograph showing a section of a solar cell that is manufactured by laminating an n layer 3, i layer 4 and p layer 5 of xcexcc-Si, in this order, on a lower electrode 2 of metal film at a substrate temperature of about 85xc2x0 C.
Although microcrystals are formed in the initial stage of the film formation due to the low substrate temperature, the crystal grain size is small over the entire cell. Therefore, the Eff of the cell was 1.5%. This is lower than the Eff in the case where the layers are laminated at the substrate temperature of 250xc2x0 C., as shown in FIG. 15. As previously stated, the formation of a mc-Si film at high temperature results in the formation of a noncrystalline film at the initial stage of film formation. This formation of the non-crystalline film badly affects the cell characteristics in such a manner as to lower FF, Jsc, and the like. Further, even if a low substrate temperature were used to form microcrystals from the initial stage of the film formation, the resulting cell characteristics still deteriorate due to the small crystal grain size produced over the entire cell area.
It is therefore an object of the present invention to provide a non-single-crystal thin film solar cell manufacturing method, which prevents the formation of non-crystalline film at the initial stage of the film formation, maintains a large grain size of microcrystals and improves the total cell efficiency without causing deterioration of characteristics such as FF and Jsc.
The above object can be accomplished by providing a method of manufacturing a non-single-crystal thin film solar cell, which comprises laminating together (a) a first-conductivity-type layer comprising mainly microcrystalline phases, (b) a substantially-intrinsic i-type semiconductor layer composed mainly of microcrystalline phases, and (c) a second-conductivity-type layer of a reverse-conductivity-type to the first-conductivity-type layer, to form a lamination layer comprising mainly microcrystalline phases, on a substrate coated with conductive film, and forming a transparent electrode and a metal grid electrode, the method comprising the steps of: (1) forming a part of the lamination layer at a first substrate temperature, and (2) forming the rest of the laminate thereon at a second substrate temperature higher than the first substrate temperature.
The above object can also be accomplished by providing a method of manufacturing a non-single-crystal thin film solar cell, which comprises laminating together (a) a first-conductivity-type layer composed mainly of microcrystalline phases, (b) a substantially-intrinsic i-type semiconductor layer composed mainly of microcrystalline phases and (c) a second-conductivity-type layer of a reverse-conductivity-type to the first-conductivity-type layer, to form a lamination layer comprising mainly microcrystalline phases, on a transparent substrate coated with conductive transparent film, and forming a metal electrode, the method comprising the steps of: (1) forming a part of the lamination layer at a first substrate temperature, and (2) forming the rest of the lamination layer thereon at a second substrate temperature higher than the first substrate temperature.
In one preferred form of the present invention, the first-conductivity-type layer is formed at the first substrate temperature, and the i type semiconductor layer is then formed at the second substrate temperature higher than the first substrate temperature. In another preferred mode of the present invention, a part of the first-conductivity-type layer is formed at the first substrate temperature, the rest of the first-conductivity-type layer is formed at the second substrate temperature higher than the first substrate temperature, and the i type semiconductor layer is then formed at a higher substrate temperature than the first substrate temperature.
In another preferred form of the present invention, the first-conductivity-type layer is formed at a lower substrate temperature than the second substrate temperature, a part of the i-type semiconductor layer is formed at the first substrate temperature, and the rest of the i-type semiconductor layer is formed at the second substrate temperature higher than the first substrate temperature.
The above object can also be accomplished by providing a method of manufacturing a non-single-crystal thin film solar cell, which comprises laminating together a first-conductivity-type layer composed mainly of microcrystalline phases and a second-conductivity-type layer of a reverse-conductivity-type to the first-conductivity-type layer, to form a lamination layer comprising mainly microcrystalline phases, on a substrate coated with conductive film, and forming a transparent electrode and a grid electrode, the method comprising the steps of: (a) forming a part of the lamination layer at a first substrate temperature, and (b) forming the rest of the lamination layer thereon at a second substrate temperature higher than the first substrate temperature.
In one preferred mode of the present invention, (a) the first-conductivity-type layer and a part of the second-conductivity-type layer is formed at a first substrate temperature lower than the second substrate; and (b) the rest of the second-conductivity-type layer is then formed at the second substrate temperature, which is higher than the first substrate temperature.